The two sides tried to build on that success the following year. In December 2004, while Lakhani was still on trial, FBI director Robert Mueller paid a visit to FSB headquarters on Moscow’s Lubyanka Square for a meeting with his Russian counterpart, Nikolai Patrushev, an old friend of President Vladimir Putin from their early days as agents in the Soviet KGB. For the press, Mueller and Patrushev staged a signing ceremony for a memorandum of cooperation between their two agencies. “First of all, this means fighting against international terrorism,” Patrushev said at the ceremony. Mueller added that the agreement would allow for the exchange of information to combat “those who threaten our countries.”

This honeymoon, however, proved short-lived. The year after Lakhani’s 2005 conviction, a former Russian spy named Alexander Litvinenko, a fervent critic of Putin and the FSB, was poisoned to death in London with a highly radioactive isotope of polonium. British investigators suspected Russian security agents of being involved in the murder. But the FSB refused to cooperate, and Russia refused to extradite the prime suspect, Andrei Lugovoi, who was also a former Russian security agent. “The exchange of information pretty much stopped after that,” says Soldatov, co-author of The New Nobility, a history of the FSB.

After the election of President Barack Obama in 2008, the U.S. tried to restore the relationship. During his first state visit to Moscow in 2009, he created the so-called U.S.-Russia Bilateral Presidential Commission with his Russian counterpart, then President Dmitri Medvedev. The commission included more than a dozen working groups, including one on international security and another one on counterterrorism. The idea was to create direct points of contact between the two governments on various issues of mutual benefit.

That pragmatic effort, which was at the center of Obama’s “reset” in relations with Russia, fell victim soon after to politics. After four years as Prime Minister, Putin decided to return to the presidency in 2012, and his re-election campaign put anti-Americanism at the center of his platform. That may have helped rally his conservative electorate, but it made practical ties a lot more difficult, especially after the U.S. began to take a harder line on rights abuses in Russia. In December, the U.S. passed the Magnitsky Act, named after a Russian whistle-blower who died in police custody, which imposes sanctions on Russian officials implicated in violating human rights.

Moscow was furious. Russia banned U.S. citizens from adopting Russian children as of Jan. 1. It also came up with its own blacklist of U.S. officials accused of rights violations, such as the former heads of the CIA prison at Guantánamo Bay and numerous U.S. law-enforcement officials. On Jan. 25, the U.S. announced it was pulling out of one of working group within the Obama-Medvedev commission, citing a state crackdown on Russian civil society. Less than a week later, the Foreign Ministry said Russia was also canceling a 2002 agreement to cooperate in the field of security and police work. Both moves were largely symbolic, but they put further political pressure on officials from both countries who had been trying to work together on issues such as terrorism and organized crime.

In this atmosphere, it may have been harder for the FSB and FBI to pursue Tsarnaev’s case jointly and share information openly. Some points of contact were still available to them. There are, for instance, accredited agents of the FBI attached to the U.S. embassy in Moscow and agents of the FSB accredited to work in Washington. “But the existence of these channels does not mean that information is passing through them,” says Soldatov. Indeed, from watching Russian state television or listening to statements on Capital Hill, it would be easy to get the impression these days that the FSB and FBI are engaged in battle, not cooperation.

That picture now seems to be changing. On Friday, Putin and Obama spoke on the phone, promising to continue joint efforts in fighting terrorism. That will add to the pressure on the FBI to explain why it did not pursue the FSB’s 2011 request about Tsarnaev more vigorously and to ensure such apparent lapses do not happen again. But even aside from the questions of blame and hindsight, the FBI will need the FSB’s help in its investigation of the Boston bombings. So far this week, the FSB appears to be a gracious and obliging host to its colleagues from across the ocean. But, says Soldatov: “The Americans are more interested in this cooperation right now than the Russians, and they could be asked to make concessions in order to renew those links.”

That could mean softening U.S. criticism of democracy and rights abuses in Russia or reining in the sanctions imposed by the Magnitsky Act. The U.S. has certainly made such concessions in the past. After the terrorist attacks of Sept. 11, 2001, it began to take a softer line on various oppressive regimes in order to get their cooperation in the war on terror. Muammar Gaddafi of Libya and Hosni Mubarak of Egypt come to mind, and neither of those relationships ended all that well. But in the wake of the Boston tragedy, the U.S. needs Russia’s help, and experience suggests that it will have to pay a diplomatic price for it. So it seems a bit early to celebrate a new era of friendship between the FSB and FBI. This week’s hospitality could yet devolve into another round of political horse trading.